Method of forming a piezoelectric ultrasonic transducer



N. F. FOSTER Re- 26,829

METHOD OF FRMING A PIEZOELECTRIC ULTRASONIC TRANSDUCER Original FiledAug. 6, 1964 March 17, 1970 /NVE/VTOR ATTORNEY /Q 25 v\.?4 QUAS/ TRA NSVERSE QUASI LONGITUD/NAL BVMFFOSTER WAVES United States Patent OfficeReissued Mar. 17, 1970 26,829 METHOD OF FORMING A PIEZOELECTRICULTRASONIC TRANSDUCER Norman F. Foster, Allentown, Pa., assignor to BellTelephone Laboratories, Incorporated, New York, N.Y., a corporation ofNew York Original No. 3,388,002, dated June 11, 1968, Ser. No. 387,837,Aug. 6, 1964, which is a continuation-impart of application Ser. No.320,379, Oct. 31, 1963. Application for reissue Oct. 24, 1968, Ser. No.771,685

Int. Cl. H01l3/16;H01v 7/02; B44d 1/18 U.S. Cl. 117-217 10 Claims Matterenclosed in heavy brackets appears in the original patent but forms nopart of this reissue specification; matter printed in italics indicatesthe additions made by reissue.

ABSTRACT OF THE DISCLOSURE A piezoelectric ultrasonic transducer isformed by evaporating a semiconductive material, such as cadmiumsulfide, having latent piezoelectric properties onto a heated substratewhere it recrystallizes into piezoelectrically aligned crystals. Theresistivity of the layer is raised so it can support a piezoelectricfield. Chose of substrate and direction of evaporation controls theprimary ultrasonic mode generated.

This invention is a continuation in part of my copending applicationSer. No. 320,379, tiled Oct. 31, 1963 and relates to piezoelectrictransducers for use with ultrasonic delay lines. More particularly, itrelates to transducers fabricated from high resistivity piezoelectricsemiconductive materials and to the method of fabricating suchtransducers.

Recently considerable attention has been given to the latentpiezoelectric properties of semiconductive materials. These materialsinclude the hexagonal semiconductive compounds of Group II-VI such ascadmium sulfide and zinc oxide. In order that these latent propertiesmay manifest themselves, at least three different parameters of thematerials must be particularly controlled. These parameters include thesize of the crystals of the material, the orientation of thepiezoelectric axis of each crystal both in terms of alignment With othercrystals and with the vibration required for the ultrasonic modedesired, and nally, the resistivity of the material which must be highenough that the piezoelectric held is not shorted out. These parametersdo not naturally occur in the proper combination to produce asubstantial piezoelectric phenomenon, which explains Why thepiezoelectric properties of these materials have only recently beenobserved.

In the copending application of D. L. White Ser. No. 208,185, tiled July3, 1962, suitable transducer layers of semiconductive material aredescribed in which the crystallographic parameters of the layer aredetermined by the crystallographic properties of a substrate upon whichor from which the layer is formed. The resistivity of the layer isdetermined by controlling its impurity content during its formation.While transducers thus formed appear to have potentialities atmoderately high frequencies, at frequencies above 100 megacycles thepresence of the substrate becomes a disadvantage. Since this substratewas chosen for its crystallographic compatibility with the transducermaterial it is unlikely to have optimum acoustical properties.Furthermore, at these high frequencies both the resistivity of thesubstrate, and the bond required to fasten it to an associated delaymedium become disadvantages.

It is therefore an object of the present invention to improve ultrasonicsemiconductive piezoelectric transducers.

It is a more specific object to form a layer of oriented highresistivity semiconductive material of moderately large crystals upon anultrasonic delay medium having any predetermined and desired acousticalproperties.

ln accordance with the invention it has been discovered that a layerproduced by evaporative techniques upon a thin metallic substrate canexhibit piezoelectric activity if particular conditions are maintainedduring the evaporative process and if particular treatment processes arefollowed after the layer is formed. In particular it has been recognizedthat when a material such as cadmium sulfide is evaporated upon ametallic substrate that has been heated and held heated duringevaporation, the cadmium sullide tends to deposit on the hot substratein a crystalline state with crystals of moderate size and with thehexagonal axes of the majority of these crystals aligned With thedirection in which the deposited material arrives at the substrate. Thiscrystalline material, however, has a resistivity too low to support asuitable piezoelectric field. In accordance with the invention theresistivity of the semiconductive layer is increased by doping duringevaporation, diffusing after evaporation, or otherwise adding a materialof the type which when introduced into the layer adds impurities whichtend to trap or compensate the current carriers of the material withoutitself introducing other current carriers. In accordance with a specificfeature of the invention, the resistivity is increased by formingadjacent to the semiconductive layer, a layer of conductive material ofthe compensating type either as the substrate or as an overplating ofthe layer or both or by adding this compensating material at the timethe deposit is formed. Thus, when the semiconductive layer and thecompensating material are heated together some of the conductivematerial will diffuse into the semiconductive layer and raise itsresistivity to the desired value. Gold, copper or silver are suitable asconductive materials for this purpose.

Other features of. the invention reside in ways in which the orientationof the piezoelectric axes of the crystals is controlled to control thedistribution of the characteristic mode of vibration of the transducerbetween shear and longitudinal mode components. In general, it has beenfound that in addition to its dependence upon the direction of arrivalof the deposited material, the orientation depends upon the substratematerial and the nature of a subsequent heat treatment. Predominantlongitudinal mode vibration is produced by slow evaporation in adirection substantially normal to a hot gold substrate. A combination ofshear and longitudinal mode vibration is produced by evaporation upon ahot copper substrate followed by a heat treatment. In this case the heattreatment has the effect of causing the orientation of the piezoelectricaxes of the crystals on the copper substrate to tip away from itsinitial position by an amount which depends on the intensity of the heattreatment. Thus, both the longitudinal mode of ultrasonic vibrationproduced by the component of the axis perpendicular to the substrate andthe shear mode produced by the component parallel to the substrate aresimultaneously generated. Finally, predominant shear mode vibration isproduced by a relatively more rapid evaporation at an acute angle to arelatively cooler silver substrate. The resulting inclination of thepiezoelectric axis produces a large shear mode component.

In accordance with a further feature of the invention, the residualcomponent of one or the other of these modes can be suppressed byforming the transducer upon an anisotropic delay mediurn so orientedthat the desired mode propagates along the delay medium while theundesired one is deflected toward the boundaries of the medium where itis scattered or absorbed.

These and other objects and features, the nature of the presentinvention and its various advantages, will appear more fully uponconsideration of the specific illustrative embodiments shown in theaccompanying drawings and described in detail in the followingexplanation of these drawings.

In the drawings:

FIGS. 1A and 1B are cross-sectional views of longitudinal and shear wavetransducers, respectively, utilizing evaporated layers of highresistivity piezoelectric material in accordance with the invention;

FIG. 2 illustrates the transducer of FIG. l in combination with a modefilter, in accordance with the invention, for producing a purelongitudinal ultrasonic vibration; and

FIG. 3 illustrates the transducer of FIG. 1 in combina- -x tion with amode filter, in accordance with the invention, for producing a puretransverse ultrasonic vibration.

More particularly, FIG. 1A represents the end of a typical delay line 15within which it is desired to launch longitudinal mode ultrasonicvibrations traveling in a direction parallel to its axis 14. Line 15 maybe of quartz, glass or a metal such as aluminum and may have anycross-sectional shape and dimensions. A first layer or film 10 issuitably plated, deposited or otherwise applied by known techniques toan end face of line 15 that is substantially normal to axis 14. Layer 10may be a conductive material selected from the group including gold,silver and copper, these being known materials that trap currentcarriers in materials such as cadmium sulfide. However, for longitudinalmode generation it appears preferable that layer 10 be formed from goldfor reasons to be set out hereinafter. Depending upon the material ofline 1S, a known flux such as Nichrome may be included between layer 10and the material of line 15 to facilitate a bond. Layer 11 representsthe semiconductive, piezoelectric material formed according to theevaporative process described hereinafter with the evaporant sourcelocated away from substrate 10 in a direction represented by the arrow16 normal to the surface of layer 10. Layer 12 represents a secondconductive layer applied over layer 11 and comprises the other electrodeof the transducer by means of which an electric field is set up in layer11 in response to alternating-current signals from source 13 appliedbetween layers l0 and 12.

In accordance with the invention, layer 10 is formed by the particularevaporative technique now to be described. To simplify the description,attention will first be directed specifically to a fabrication of alongitudinal mode transducer as shown in FIG. 1A employing hexagy onalcadmium sulfide as the preferred semiconductive material, it beingunderstood that similar compounds would be handled in related ways. Forexample, other materials having piezoelectric, semiconductive propertiesof Group II-IV and having either a hexagonal or wurtzite structure maybe used to practice the invention. Specific examples in this class arezinc oxide, cadmium selenide, zinc sulfide, and magnesium telluride. Inaddition, cubic Group II-IV materials such as zinc sulfide (zinc blend),cubic cadmium sulfide and cubic zinc oxide may be employed.

The evaporative procedure involves the use of an evaporator of the typein which the boat containing the evaporant and the jig holding thesubstrate structure may be separately maintained at differenttemperatures within a controllable atmosphere. Evaporation is thereforedefined as a process in which energy such as heat is applied to :tsource of evaporant to cause portions of the source material to bedriven away from the source in submicroscopic particles. Suchevaporators are readily commercially available.

Powdered cadmium sulfide is first placed in the boat of the vaporatorand heated to a dull red heat for a few minutes in a vacuum. This stepis merely precautionary and allows foreign material in the form ofgasses to be driven from the cadmium sulfide. Line 1S, upon which goldlayer l0 has already been formed, is placed in the evaporator with layer10 a few inches from the boat containing the cadmium sulfide and locatedso that layer 10 which constitutes the substrate upon which theevaporated film is deposited is normal to direction from the boat. Theevaporator is evacuated, a pressure of from 2 10`6 to 6 l0-6 torr beingsatisfactory. The substrate is then heated to a temperature sufficientlyhigh to drive ol foreign material and other contamination. The cadmiumsulfide is then heated to a temperature which causes it to evaporate. Atemperature in the range of 750 to 900 C. has proven satisfactoryalthough this temperature has not been found to be critical. Thesubstrate (layer 10) is simultaneously brought to a temperature highenough that the deposited material forms upon it in a crystalline state.A temperature of at least 180 C. and preferably in the range of 200 to230 C. has proven satisfactory although substrate temperatures abovethis will produce acceptable results so long as they are sufficientlybelow the evaporation temperature of the material to be deposited toprevent undue re-evaporation. Temperatures much `below 180 C. cause thedeposited material to form in an amorphous and disordered state. Ingeneral, it has been found that the evaporant and substrate temperaturesshould have such a relationship to each other that the deposited layerbuilds up at a rate of less than one micron per minute. Rates muchgreater than this tend to produce less perfect crystal structures. Thetotal length of time of course depends upon the thickness desired forlayer 11 which in turn depends upon the intended operating frequency.

When an appropriate layer has been builit up, the temperature of layers10 and 11 is raised to one substantially above that maintained duringevaporation and held in an inert atmosphere for a time selectedaccording to known current carrier compensating principles in order toraise the resistivity of layer 11 to at least 10*i ohms/cm. Atemperature of approximately 450 C. for la period of approximatelyone-quarter of an hour has proven satisfactory. Alternatively, currentcarrier compensating atoms of silver, gold or copper may be depositedalong with the deposited semiconductive material during the vaporationprocess in which case the length of time and temperature required toattain the proper resistivity is reduced or eliminated.

The transducer is completed by adding a second conductive layer 12 uponthe surface of layer 11 and suitably attaching conductors to both layers10 and 12.

If instead of gold as the material for substrate 10, copper is employed,it has been found that the heat treatment following evaporation causesthe orientation of the piezoelectric axes of the crystals to tip awayfrom the normal by amounts which depend on the intensity of the heattreatment and that a substantial shear mode component is produced alongwith a substantial longitudinal mode component. The presence of bothmodes is useful in an application in which it is desired to produce twosignals at precisely spaced times after an input signal. Thus, the inputsignal from source 13 starts both longitudinal and shear modes travelingtoward the output end of the delay line at different characteristicvelocities to arrive at the output `at different times.

Should it be desired to accentuate one or the other of these modes thefollowing considerations should be taken into account. The tilt angleappears to be dependent upon the severity of the subsequent heattreatment. Therefore, for a smaller angle `and a larger longitudinalmode component, lower temperatures and shorter times are preferable. Forlarger shear wave components, higher temperatures and longer timesshould be used. In

addition, an over-plating 0f copper as electrode 12 in addition to asubstrate of copper both applied before subsequent heat treatmentincreases the axis rotation.

In order to generate an even larger shear wave component, themodification shown in FIG. 1B should be used. In FIG. 1B referencenumerals corresponding to those of FIG. 1A have been employed todesignate corresponding components. Modification will be seen to residein the fact that substrate layer 18 (corresponding to 10 of FIG. 1A) ispreferably formed of silver, and that layer 19 representing thesemiconductive, piezo electric layer is deposited from an evaporantsource located away from substrate 18 in a direction represented by thearrow 17 which makes an acute angle with the substrate. The evaporativetechnique described above for FIG. 1A may be substantially followedexcept that a lower substrate temperature in the range of from 17o-200C. has proven desirable.

While there is no intent to limit the scope of the present invention bythe theory now to be presented, this theory is believed to be accurateand consistent with observable facts and accepted scientific principles.Thus, it appears that when the vaporized cadmium sulde is deposited uponthe heated substrate, the first material deposited is in the form ofrandomly oriented crystals of small size. As further material isdeposited, those crystals which have their hexagonal axes aligned withthe direction in which the new material arrives tend to recrystallizeand grow. If this direction is substantially normal to the surface ofthe substrate as in FIG 1A, the majority of crystals which grow tomoderate size have their axes perpendicular to this surface. If,however, this direction is at an acute angle to the substrate as in FIG.1B, the crystals tend to grow at acute angles. It has been determinedexperimentally that crystals tend to grow more rapidly in a normaldirection on a gold substrate and more rapidly at an angle on a silversubstrate A possible explanation of this difference resides in the smallsurface mobility that cadmium sulde has on silver with which it has astrong chemical bond and the corresponding large surface mobility ongold with which there is a weaker chemical bond. On the other hand whenthe substrate is copper the subsequent heat treatment tends to tilt theaxes of the majority of the crystals away from their initial orientationto a much greater extent than with either silver or gold. Thisphenomenon has been recognized in the art and has been designated as theCakenberghe effect even though the reasons underlying it have not beenfully explained. Therefore, gold substrate 10 is preferred for thelongitudinal wave embodiment of FIG. 1A, a copper substrate for a mixedmode embodiment and a silver substrate for the shear wave embodiment ofFIG. 1B.

Regardless of substrate, the formed layer is initially of too low aresistivity to support a satisfactory piezoelectric field. According toa first alternative the resistivity is raised without a previousaddition of compensating material by the subsequent heat treatment. Itis believed that this increase in resistivity cornes about jointly froma diffusion into the material of compensating atoms from the substrateand/or oxygen atoms from the surrounding atmosphere which tend to trap,compensate or otherwise neutralize current carriers resulting fromexcess cadmium in the deposited material. During this heat treatment theaxes of the majority of the crystals may be somewhat tilted away fromperpendicular as described above. Alternatively, the resistivity of thelayer may be increased by evaporating the compensating atoms along withthe semiconductive material or by applying the overlayer 12 ofcompensating material before the subsequent heat treatment to provide asource of compensating atoms. Alternatively or in combination withcompensation, the resistivity of the layer may be increased by renderingit more nearly stoichiometric. For example, in the specific case ofcadmium sulfide where the low resistivity of the Cil evaporated layerappears to result from an excess of cadmium which supplies the currentcarriers, these may be eliminated by heating the layer in a vacuum todrive off the excess cadmium or in air or sulphur vapor to fill thesulphur voids.

Regardless of the method of rendering the piezoelectric layer highlyresistive, the piezoelectric axis is never completely correctly aligned.Thus, when a signal from source 13 is applied between electrodes 10 and12, a shear wave or a wave having transverse vibrating components isproduced by the component normal to axis 14 and a wave havinglongitudinal vibrating components in produced by the component parallelto axis 14.

Discrimination can be obtained between the modes on the basis offrequency. For a given transducer there is a center frequency range ofoperation in which both longitudinal and shear modes are produced withrelatively equal eftciency. At frequencies in a range above this latterrange the efficiency for the longitudinal mode markedly improves whilethe efficiency for the shear mode decreases. Conversely, at frequenciesbelow this range etliciency for the shear mode increases and eiciencyfor the longitudinal mode decreases.

In the event that further mode separation is desired, the mode filtercombination now to be described with respect to the embodiments of FIGS.2 and 3 may be employed. In both embodiments use is made of the modeselective propagation properties of anisotropic material, i.e., materialin which the elastic moduli changes with orientation relative to thecrystal axes. In these materials there are limited directions in which apure longitudinal wave or a pure shear wave can be propagated. In otherdirections quasi longitudinal or quasi shear waves are propagated indirections which make angles to the major surfaces of the crystal. Whileseveral examples could be given with materials having trigonal, cubicand hexagonal crystals, a single example for each mode in terms ofquartz, a trigonal crystal, will serve to illustrate the invention. Fora discussion of the large number of cuts having different orientationswith respect to the crystal axes of quartz together with a detaileddescription of the conventional designation of these cuts, reference maybe had to either of the texts of W. P. Mason entitled ElectromechanicalTransducers and Wave Filters or Piezoelectric Crystals and TheirApplication to Ultrasonics, or the text of R. A. Heising entitled QuartzCrystals for Electrical Circuits, all published by D. Van NostrandCompany, Inc. of New York.

Referring more particularly to FIG. 2, the transducer comprising layers10, 11 and 12 is formed according to the process described heretoforeupon a bar 20 cut from a single crystal of quartz and upon a facethereof that is normal to the Z or optic axis of the crystal asrepresented by arrow 21. Such a member is known as a Z-cut bar. Bar 20may comprise the whole delay line or it may be interposed between thetransducer 10-11-12 and a delay line 22.

Waves having both a direction of propagation and a particle motion inthe Z direction, i.e., longitudinal waves as hereinabove defined, have amaximum energy flux vector lying along the Z axis. They, therefore,emerge from member 2l) with little loss and enter delay line 22.However, waves which have a particle motion normal to the Z axis, i.e.,transverse or shear waves, have a maximum energy flux vector at an angleof substantially 16 to the Z axis so that the vector describes a cone asit is rotated about the Z axis. The term conical internal refraction hasbeen applied to this situation. Thus, nonlongitudinal energy from theface of the transducer is directed as quasi transverse waves, alongpaths generally designated by the shaded areas 23 and 24 to mpinge uponthe side boundaries of crystal section 20. These boundaries are madeenergy dissipative, either by roughening the surface thereof to scatterwave energy or by loading this surface with acoustical absorbingmaterial as represented on FIG. 2 by 25 or both. It should be understoodthat axes equivalent to the Z axis will have similar properties.

In FIG. 3 shear or transverse waves are passed to the exclusion oflongitudinal waves by a BC cut bar 30 of single crystal quartz. As shownby vector symbol 33 the BC axis is that axis at an angle ofsubstantially 31 from the Z or optical axis toward the Y or mechanicalaxis rotated about the X or electrical axis (extending into the paper inFIG. 3). Transducer 10-11-12 is located upon the face of the crystalnormal to the BC axis and surface 32 parallel to the axis is madedissipative as in FIG. 2. Shear or transverse modes propagate withoutinterference parallel to the BC axis to the connected delay line 22.However, waves having a longitudinal particle motion have a maximumenergy flux vector substantially 5 away from the BC axis toward the Zaxis. Thus, nontransverse energy from the face f the transducer isdirected as quasi longitudinal waves along paths generally designated bythe shaded area 34 to impinge upon the side boundary of section 30through which the Z axis passes and is there dissipated by beingscattered or absorbed by surface 32. It should be understood, of course,

that bars cut along equivalent axes such as the AC will have similarproperties to a BC cut bar.

In all cases it is to be understood that the abovedescribed arrangementsare merely illustrative of a small number of many possible applicationsof the principles of the invention. Numerous and varied otherarrangements in accordance with these principles may readily be devisedby those skilled in the art without departing from the spirit and scopeof the invention.

What is claimed is:

1. The method of forming an ultrasonic transducer from semiconductivematerial having latent piezoelectric properties which comprises applyingenergy to a source body of said material suflicient to cause portions ofsaid material to be driven away from said source body in submicroscopicparticles, locating a substrate in the path of said portions driven awaywhereby said portions form a layer on said substrate, heating saidsubstrate while said layer is being formed to a first temperature belowthe evaporation temperature of said material but high enough that saidlayer material forms in a crystalline state with the piezoelectric axesof a majority of crystals aligned and polarized in the same direction,and introducing further material to said layer which compensates thecurrent carriers in said layer material to increase the resistivitythereof high enough that a piezoelectric eld a may be supported by saidlayer.

2. The method of claim 1 wherein said source material is heated to causeit to be evaporated onto said substrate and wherein said substrate isheated in the presence of material which compensates said currentcarriers.

3. The method of claim 1 wherein said substrate is formed of a materialfrom the group consisting of copper, gold and silver, and wherein acompound from the group II-VI is applied by evaporation onto saidsubstrate.

4. The method of claim 3 wherein said substrate is formed of gold andwherein said compound is evaporated on said substrate in a directionsubstantially normal to said substrate.

5. The method of claim 3 wherein said substrate is formed of silver andwherein said compound is evaporated on said substrate in a direction ata single acute angle to said substrate so that all material forming saidlayer arrives at said substrate at said angle to control the directionin which said majority of crystals are aligned.

6. The method of claim 3 wherein said compensating material isintroduced by evaporating said compensaating material along with saidlayer material.

7. The method of claim 3 wherein said compensating material inintroduced by further heating said layer and said substrate at atemperature substantially above said first temperature until materialfrom said substrate diffuses into said layer to compensate currentcarriers in said layer material.

8. The method of forming an ultrasonic transducer from semiconductivepiezoelectric material which comprises forming a substrate of copper,evaporating cadmium sulfide onto said substrate to form a layer on saidsubstrate, maintaining said substrate at a temperature duringevaporation of at least C., further heating said layer and saidsubstrate at a temperature of at least 250 C. until copper from saidsubstrate diffuses into said layer to compensate current carriers insaid cadmium sulfide, and forming a conductive layer upon the face ofsaid cadmium sulfide opposite said substrate.

9. The method of forming an ultrasonic transducer from sernconductivematerial having latent piezoelectric properties which comprises applyingenergy to a source body of said material suicient to cause portions ofsaid material to be driven away from said source body in subrnicroscopicparticles, locating a substrate in the path of said portions driven awaywhereby said portions form a loyer on said substrate in a crystallinestate with the piezoelectric axes o] a majority of the crystals alignedund polarized in the same direction, the plane of said layer being atanacute angle to said path so that material fortning said layer arrives atsaid substrate along said path ut said angle to control the direction iuwhich said crystals are aligned and polarized, and controlling thecomposition of said layer to produce a resistivity high enough that apiezoelectric field may be supported by said layer.

10. Tlze method of claim 9, wherein said substrate is formed from aimaterial from the group consisting of copper, gold and silver, undwherein a compound from the group lI-Vl forms said layer on saidsubstrate.

References Cited The following references, cited by the Examiner, are ofrecord in the patented tile of this patent or the original patent.

UNITED STATES PATENTS 2,759,861 8/1956 Collins et al. 14S-1.5

2,938,816 5/1960 Gunther 117-212 3,065,112 11/1962 Gilles et al. 117-200FOREIGN PATENTS 1,057,845 5/1959 Germany.

OTHER REFERENCES Journal of Applied Physics, Dresner et aL, vol. 34, No.8, August 1963. pp. 2390-2395.

WILLIAM L. JARVIS, Primary Examiner U.S. Cl. X.R. 117-106, 215

